Bodies capable of conducting electricity, including bodies made entirely of metal and bodies having both metallic and nonmetallic portions, often have outer surfaces that need to be cleaned. Rust, scale, smut, petroleum derived contaminants, flux, carbonization, nonmetallic coatings, corrosion, dirt and the like may form or be deposited on the surface of the body. These surface deposits or contaminants must be removed so that the body may be recycled and reused or to prepare the body for subsequent surface treatment. Examples of metallic bodies that may require cleaning include grocery carts, metallic brake shoes, jewelry, and electronic circuit boards.
An early method of cleaning metal bodies immerses the bodies in a high temperature cyanide bath. Major ingredients of the cyanide bath include caustic soda and sodium cyanide or potassium cyanide. The bath is heated to a temperature in excess of 700 degrees F. All of the cyanide and alkali materials are limited in life and have to be discarded and entirely fresh bath solutions made. The cyanide bath has the potential of liberating deadly cyanide gas, and the cyanide bath itself is a hazardous waste that requires special and expensive waste treatment and disposal.
To overcome the disadvantages of the cyanide bath, a variety of electrolytic cleaning systems were developed. Many of these systems use caustic soda (NaOH) to form a highly alkaline caustic soda bath. Caustic soda attacks galvanized steel, brass, bronze, copper, aluminum, magnesium, titanium and other metals. The caustic soda attacks the metal itself. Even if the metal could withstand immersion in the caustic soda bath, subsequent brushing or spraying treatments may be needed to remove tenacious impurities. The caustic soda bath is highly corrosive and requires special care in handling and disposal.
Other electrolytic cleaning systems have been proposed. One electrolytic system uses an electrolyte solution containing ferric sulfate and ammonium bifluoride. This system may generate objectionable fumes. The system does not de-scale or de-smut. A sludge containing insoluble salts of such metals as aluminum, copper, brass and bronze is generated that must be disposed of as a hazardous waste. Another electrolytic system uses an electrolyte solution composed of a phosphate alkaline material heated to 160-190 degrees F. Metallic ions, such as lead, tin, zinc or cadmium ions act as catalysts in the solution for removal of scale from stainless steel. These metallic ions remain in the spent electrolyte and require hazardous waste treatment and disposal. A film of metal may be deposited on the surface of the treated object. The film may be acceptable in cleaning stainless steel, but would be totally unacceptable in cleaning surfaces of other metallic objects such as circuit boards.
Yet another known method of cleaning is directed to consumer cleaning of gold, silver, coins and jewelry. The object to be cleaned is immersed in an electrolyte and a relatively low voltage and amperage electric current is passed through the electrolyte solution. The method is designed specifically to remove tarnish, which are sulfides of gold and silver. During cleaning, hydrogen sulfide gas is created. Hydrogen sulfide is a noxious, poisonous gas that would present a serious problem in commercial operation.
The above conventional methods of cleaning metallic bodies require extremely high operating temperatures, toxic chemicals or highly corrosive liquids. These conventional methods are designed primarily to remove rust, scale or smut from iron or steel bodies, and are not suitable for cleaning of other types of metallic bodies. The methods generate hazardous wastes that must be disposed of in compliance with environmental regulations and at high cost. Immersing the metallic body in the electrolyte may also be inefficient, as only a small number of bodies may be treated at a time.
Thus, there is a need for an improved method and apparatus for conductive cleaning bodies. The improved method should clean a wide variety of bodies, and should not be limited to iron or steel. The improved method should use non-toxic materials that are non-hazardous to personnel and should not require special disposal. The improved method should be efficient and allow the cleaning of a large number of bodies at the same time. Treatment to clean the bodies should not harm the bodies.